An extrusion system having a flat-sheet die exhibiting a die design known from plastics technology is generally used for extrusion of polymer-bonded ceramic flat sections, i.e., films and green films. As shown in FIG. 1, such dies have an inlet, a distributor channel including a throttle field and a die outlet, i.e., a die relief having a variable gap height, i.e., a flex-lip, using which the thickness of the extruded flat sections is adjustable.
However, extrusion of ceramic sections in particular differs fundamentally from extrusion of plastics in two regards. First, the viscosity (i.e., the intrinsic viscosity in the case of intrinsically viscous substances, in which the viscosity depends on the shear rate) of extruded ceramic pastes is greater than that of plastics, and furthermore, a ceramic paste generally has a flow limit. In addition, in extrusion of flat ceramic sections, the fact that subsequent processing steps such as removal of binders or sintering may be very sensitive in their response to stresses frozen into the extruded flat sections should be taken into account.
FIG. 2 illustrates the flow velocity profile of a ceramic paste extruded through a conventional flat-sheet die in the outlet of the die for various viscosities of the extruded ceramic pastes, showing that the rate of flow is zero at the edges of the die and it is maximum at the center. The viscosity or intrinsic viscosity of the extruded ceramic paste is characterized by the flow exponent, which is defined by:{dot over (γ)}=Φτmwhere {dot over (γ)} is the gradient of the flow velocity, i.e., the shear rate in the extruded paste, Φ is the fluidity and τ is the shear stress. Accordingly, the following equation holds for the viscosity, i.e., intrinsic viscosity η of the ceramic paste:
  η  =            Φ              -                  1          m                      ⁢                            γ          .                          (                                    1              m                        -            1                    )                    .      
FIG. 2 shows that shear rate {dot over (γ)} is zero at the center point of the die outlet and is maximum at the edge. The shear rate at the edge of the die is {dot over (γ)}w. Shear rate {dot over (γ)}w at the wall is calculated as follows at a given volume throughput {dot over (V)}:
            γ      .        w    =      2    ⁢          (              m        +        2            )        ⁢                  V        .                              H          2                ⁢        B            where m again denotes the flow exponent of the extruded paste and is thus a measure of the intrinsic viscosity of this paste, and H denotes the height of the flat-sheet die at the site in question and B is its width.
FIG. 2 shows in particular that, when there is a flow limit, i.e., a high flow exponent m, shearing of the extruded material occurs almost exclusively in the edge area of the die. In the case of extruded polymer-bonded ceramic pastes, this shearing produces an orientation of the binder molecules added to these pastes, which may result in considerable after-shrinkage of the extruded flat sections in the case of a downstream add-on processing used on the extruded flat sections, e.g., imprinting by screen printing or lamination. Since the degree of molecular orientation established in the extruded flat sections is not constant over the thickness of this film due to the differences in shear rates, such a post-shrinkage, i.e., relaxation which is associated with a macroscopic change in shape, cannot usually take place completely. Thus, in the past there have always been residual internal stresses, which are manifested as unwanted changes in geometry (shrinkage) in subsequent printing steps or lamination steps. In addition, delamination similar to puff pastry also frequently occurs near the film surfaces during a subsequent sintering.